1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method which process the tomograms captured by a tomography apparatus.
2. Description of the Related Art
An ophthalmic tomography apparatus such as an optical coherence tomography (OCT) apparatus is configured to generate an image (or tomogram) of a retina from a signal produced through the interference between near-infrared light reflected from the retina and reference light. In general, the image quality of a tomogram generated based on interference light in this manner depends on the intensity of near-infrared light striking the retina. In order to improve the image quality of a tomogram, it is necessary to increase the intensity of near-infrared light applied to the retina. The intensity of near-infrared light which can be applied to the retina has a certain limit from the viewpoint of safety.
It is therefore required to generate tomograms with high image quality while applying near-infrared light within an intensity range which is deemed to be safe. Attempts have been made to meet this requirement, mainly based on the following two methods:
(i) a method using oversampling; and
(ii) a method using averaging.
Attempts based on these two prior art methods will be briefly described below.
The oversampling method will be described first with reference to FIG. 9. In FIG. 9, 9a shows an example of a tomogram of the retina imaged by a tomography apparatus, in which: reference symbol Ti denotes a two-dimensional tomogram (B-scan image); and Aij, a scanning line (A-scan). The two-dimensional tomogram Ti comprises a plurality of scanning lines Aij located on the same plane.
In FIG. 9, 9c shows an example of the irradiation distribution of near-infrared light applied to the retina, when viewed from the fundus surface in the depth direction of the retina, in capturing the two-dimensional tomogram Ti. In 9c of FIG. 9, ellipses Ai1 to Aim represent the diameters of near-infrared light spots.
In FIG. 9, 9b shows an example of a tomogram of the retina imaged by the tomography apparatus, more specifically a two-dimensional tomogram Ti′ obtained by imaging the same imaging range as that indicated by 9a in FIG. 9 with double the number of scanning lines. In FIG. 9, 9d shows an example of the irradiation distribution of near-infrared light applied to the retina, when viewed from the fundus surface in the depth direction of the retina, in capturing the two-dimensional tomogram Ti′. In 9d of FIG. 9, ellipses Ai1 to Ai2m represent the diameters of near-infrared light beams.
As is obvious from 9a and 9b in FIG. 9, if the imaging range remains the same, the resolution of a two-dimensional tomogram increases with an increase in the number of scanning lines. In addition, as is obvious from 9c and 9d in FIG. 9, in order to increase the resolution by increasing the number of scanning lines, it is necessary to irradiate the retina with near-infrared light such that adjacent beams overlap each other.
The method of generating a high-resolution two-dimensional tomogram by irradiating an object with adjacent beams so as to make them overlap each other is generally called the oversampling method.
The averaging method is a method of generating a tomogram with little noise by averaging and combining a plurality of tomograms captured by scanning the same imaging range with the same number of scanning lines a plurality of number of times (see, for example, Japanese Patent Laid-Open No. 2008-237238).
The two methods for generating tomograms with high image quality have the following problems. In the case of the averaging method disclosed in Japanese Patent Laid-Open No. 2008-237238, a plurality of tomograms to be averaged and combined are tomograms captured at different times. Since the pixel values of corresponding pixels of the respective tomograms are averaged, this method is effective in reducing noise contained in each tomogram. However, the resolution of each tomogram remains the same, and hence it is difficult to generate a high-resolution tomogram by combining the tomograms.
In the case of the oversampling method, it is possible to generate a tomogram with a higher resolution by increasing the number of scanning lines and increasing the overlap width. If, however, the number of scanning lines increases, the time required to capture one tomogram increases. The tomograms to be captured become susceptible to the influences of movements or flicks of the eyeballs by the patient during scanning, the movement of the head, and the like. These movements produce distortions in the captured tomograms.
In order to generate tomograms with high image quality, it is preferable to perform imaging under imaging conditions robust against the influences of movements of the eyeball, movement of the head and the like, so as to generate a high-resolution tomogram with minimum noise. On the other hand, movements of the eyeballs, movement of the head and the like vary in magnitude among different individuals; hence imaging conditions which are robust against the influences of such movements are not necessarily constant.